chore: seal Design Vision v1 and monorepo scaffold

Archive the differentiated YuXinGu product docs, AI engineering system,
design contract, and Go/Vue scaffold. Next execution prioritizes Cece-parity
over early innovation (see .ai/product/STRATEGY.md).

Co-authored-by: Cursor <cursoragent@cursor.com>

.ai/adr/0001-use-golang.md

@@ -0,0 +1,20 @@
+# ADR-0001 Use Golang for API
+
+## Status
+
+Accepted
+
+## Decision
+
+Backend is Go only (`apps/api`).
+
+## Reason
+
+1. Fits small team / AI-assisted CRUD + rule engines
+2. Simple deploy (single binary)
+3. Strong concurrency for future AI/proxy workloads
+
+## Never
+
+Never replace the API language with Node/Java/Python without a new ADR and explicit approval.  
+Legacy `server.py` is temporary and must not grow new product APIs.